A windshield, which term is here intended to cover a front, rear, or side window or windscreen of an automobile or other vehicle or anything secured thereto, is frequently made self-defrosting by imbedding in it wires that heat when electrically energized. This prevents vapor from condensing on the windshield and even frees same of ice or snow.
The principal disadvantage with this style of defrosting, as opposed to blowing a current of normally warm air over the windshield, is that it is quite slow. Dissipating enough heat electrically in the windshield to effectively defrost it is difficult because the vehicle has a limited supply of eletrical energy.
Hence it is known, as for example from German utility model No. 8,004,971 filed with a claim to an Italian priority date of Feb. 26, 1979, to subdivide the wires into several groups defining a fast-heat area and a slow-heat area. The fast-heat area is normally situated in the center of the field of view, so that this critical region can be defrosted rapidly, while the slow-heat area lies adjacent or around it where defrosting is less critical.
To achieve this effect a group of wires having a relatively low resistance, and therefore large current draw, is provided at the fast-heat zone and a group of wires of greater resistance and smaller current draw is provided at the slow-heat zone. For fast concentrated heating only the central low-resistance zone is connected up, and for slower more generalized heating the wires of the other zone are. This system works fairly well, but has the considerable disadvantage of putting a great load on the voltage source in the fast-heat mode. In the slow-heat mode the load is less, but since in cold weather, when the system is used, a vehicle battery is weakest, this poses a great load at the time when it can least well be borne.
German Pat. No. 692,313 of K. Platte describes another such system that presents a heavy load for fast heating and a smaller load for slow heating. In German Pat. No. 721,765 also of K. Platte the desirability of having the same load in different modes is recognized, but the solution puts a resistor in the circuit to equalize the load, a plain waste of energy.